Augustine

United States

59.363°N, 153.43°W; summit elev. 1252 m

All times are local (unless otherwise noted)

Observations made during an overflight on 18 January indicated that the summit was steaming vigorously, consistent with the formation of a new lava dome. Observers also noted ballistic bombs, block and ash flow deposits, and dilute-cloud surge deposits on the volcano's flanks. A white ash-poor steam plume was observed rising to about 2.6 km (8,500 feet) a.s.l. Seismicity decreased significantly over 19-20 January, but remained above background levels through 24 January. Night-time satellite views during 22-24 January showed faint thermal anomalies.

Geologic Background. Augustine volcano, rising above Kamishak Bay in the southern Cook Inlet about 290 km SW of Anchorage, is the most active volcano of the eastern Aleutian arc. It consists of a complex of overlapping summit lava domes surrounded by an apron of volcaniclastic debris that descends to the sea on all sides. Few lava flows are exposed; the flanks consist mainly of debris-avalanche and pyroclastic-flow deposits formed by repeated collapse and regrowth of the volcano's summit. The latest episode of edifice collapse occurred during Augustine's largest historical eruption in 1883; subsequent dome growth has restored the volcano to a height comparable to that prior to 1883. The oldest dated volcanic rocks on Augustine are more than 40,000 years old. At least 11 large debris avalanches have reached the sea during the past 1800-2000 years, and five major pumiceous tephras have been erupted during this interval. Historical eruptions have typically consisted of explosive activity with emplacement of pumiceous pyroclastic-flow deposits followed by lava dome extrusion with associated block-and-ash flows.